Peak current mode control is commonly used in DC-DC converters. It requires a slope compensation for stable operation when the duty cycle exceeds 50 percent. The slope compensation is added onto a sensed inductor current signal to set a limit of the inductor current. A conventional peak current mode controlled DC-DC converter is shown in FIG. 1.
As seen, there is a current sense block that senses the current flowing through a high-side switch. This current is equal to the inductor current when the high-side switch is turned on. There is also a slope compensation block providing a slope compensation signal added onto the sensed current to get a RAMP signal. The output voltage is fed back through a divider to an error amplifier. The difference between the feedback voltage VFB and the reference VREF is amplified by an error amplifier EA. The output of the error amplifier EAO is clamped by a clamp block, for example a Zener diode equivalent circuit, so as to set the limit of the peak inductor current.
The output of the error amplifier EAO and the RAMP signal is compared at a comparator COMP. At the start of each clock cycle, if RAMP signal is lower than EAO signal, a RS latch is set by the clock to turn on the high-side switch, and the inductor current starts to increase. The RAMP signal increases with the inductor current. When it increases to higher than the output of the error amplifier EAO, the comparator COMP outputs a high-level signal to reset the RS latch, so that the high-side switch in the switching stage is turned off, and a low-side switch in the switching stage is turned on. As a result, the inductor current decreases. This can be seen in FIG. 2.
As illustrated above, the sum of the slope compensation signal VSC and the sensed peak inductor current VCS is equal to the output of the error amplifier EAO through such regulation.
Since EAO level is the sum of the slope compensation signal VSC and the sensed peak inductor current VCS, for a given clamped EAO level, the actual inductor peak current varies with the amplitude of the slope compensation signal, which ultimately depends on the duty cycle. This variation is especially large or unacceptable for high duty cycle operation, where the amplitude of the slope compensation signal could be significantly larger than the sensed inductor current signal, i.e., peak of VSC>>peak of VCS.